<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        #box{width: 100px;height:100px;background: red;position: absolute;left:0;top:0;}
    </style>
</head>
<body>
    <div id="box"></div>
</body>
<script>
    
    var obox = document.getElementById("box");

    // 按下
    obox.onmousedown = function(eve){
        // 获取按下的事件对象，用来获取按下的一瞬间相对于当前元素的坐标
        var downE = eve || window.event;
        document.onmousemove = function(eve){
            var moveE = eve || window.event;

            // 计算位置：移动时相对于页面的坐标 - 按下时相对于当前元素的坐标
            var l = moveE.pageX - downE.offsetX;
            var t = moveE.pageY - downE.offsetY;

            // 边界限定
            if(l<0) l=0;
            if(t<0) t=0;
            if(l > document.documentElement.clientWidth-obox.offsetWidth){
                l = document.documentElement.clientWidth-obox.offsetWidth
            }
            if(t > document.documentElement.clientHeight-obox.offsetHeight){
                t = document.documentElement.clientHeight-obox.offsetHeight;
            }

            // 设置给元素
            obox.style.left = l + "px";
            obox.style.top = t + "px";
        }
        // 抬起
        document.onmouseup = function(){
            // 清除移动和抬起
            document.onmousemove = null;
            document.onmouseup = null;
        }
    }

</script>
</html>